Removing normal moveout from seismic traces



May 12, 195B E. P.. MEINRS, 1R Re. 24,548

REMOVING NORMAL MOVEOUT FROM SEISMIC TRACES original Filed Dec. 28,1954 2 sheets-sheet 1 -ronmavsl May 12, 1959 E. P MEINERS, JR v Re- 24,648

Rxsnvlovmc; NORMAL MovEoUT FROM sErsMrc TRAcEs 2 sheetssheet 2 Original Filed Dec.l 28, 1954 INVENTOR fowl/v P. MEI/VERS, JR.

nited States Patent M REMOVING NORMAL MOVEOUT FROM SEISMIC TRACES Edwin P. Meiners, Jr., Houston, Tex., assignor to Calii fornia Research Corporation, San Francisco, Calif., a corporation of Delaware Original No. 2,810,898, dated October 22, 1957, Serial No. 478,145, December 28, 1954. Application for reissue January 12, 1959, Serial No. 786,439

6 Claims. (Cl. 340-15) This invention relates in general to seismic prospectingand relates more specifically to methods of and apparatus for analyzing data obtained in such prospecting.

In the reflection method of seismic prospecting, energy from an artificial seismic disturbance is reflected from various subsurface strata back to seismic wave detectors at or near the surface of the earth which convert the detected movement into corresponding output signals. The output signals are amplified and then recorded for subsequent analysis. Usually, the seismic Wave detectors are spaced different distances from the location of the seismic disturbance so that energy reilected from a given reecting horizon arrives at the different detectors at different times, resulting in time shifts of corresponding signal portions of the different seismic traces. These time shifts where are caused by the dilerent spacings of the detectors are commonly referred to as normal moveout, and they tend to obscure alignments of corresponding signal portions across the traces, thus making it diicult to accurately determine the presence of a refleeting horizon. Additionally, where the seismic traces are to be combined or mixed into a single trace in which the signal portions of the mixed traces reinforce each other and the extraneous energy or noise portions tend to` randomize or cancel, the presence in the mixed traces of the normal moveout time variations prevents accurate alignment of corresponding signal portions across the traces, thus resulting in a less than optimum reinforcemen of the corresponding signal portions inthe resultant mixed trace.

The normal moveout time varies in magnitude during the seismic disturbance, being largest immediately after the disturbance, when the differences in distances of the travel paths to the different detectors for energy from a given reilecting horizon are largest, and decreasing in magnitude as these differences decrease for successively deeperreilecting horizons. The exact manner in which the normal movemout time varies as a function of the time after the disturbance will, of course, depend on the spacings of the different detectors and the particular velocity function obtaining in the surveyed area. It is customary to compute the normal moveout time for a given reecting horizon and a given detector, either on the basis of a known velocity function or from eld data obtained in the eld with split spreads. In the latter case the normal moveout for a particular reilection is given as one-half of the sum of the moveouts of the outside traces, representing equal offsets but opposite in direction.

I-Ieretofore, numerous methods and apparatusfhave been proposed for removing the normal moveout time variations from seismic traces, but none has been particularly successful. With the advent of reproducible Re. 24,648 Reissued May 12, 1959 have been proposed for removing normal moveout by eifecting relative shifts in the positions of the recording or reproducing heads relative to the recording medium to produce relative time shifts in the recorded or reproduced traces. In one of such methods, the reproducibly recorded traces are reproduced a plurality of times and the reproducing heads are shifted different amounts for each reproduction to produce normal moveout compensation for a given reflection on each reproduction. However, this method has the disadvantage that it requires a reproduction of the traces, and the consequent shifting of the reproducing heads, for every reilection of interest, thus rendering the operation very time consuming from a record processing standpoint. This method has the further disadvantage that if the reproduced traces are to be mixed, only that signal portion of each trace corresponding to the reflection for which normal moveout hasbeen removed will produce optimum reinforcement when mixed.

An additional method sometimes utilized to remove normal moveout is to continuously move each of the recording or reproducing heads relative to the recording medium during either recording or reproducingv of the traces to continuously vary the time sequences of the traces in an attempt to provide the desired correction. However, the problem is complicated by the fact that' the required normal moveout correction varies nonlinearly with respect to the time elapsing after the disturbance, the required correction being largest immediately after the disturbance, when the differences in the distances of the travel paths to the diiferent detectors for a given reflection are largest, and decreasing nonlinearly with time as these differences decrease. An additional complicating consideration is that the instantaneously values of the required correction vary nonlinearly from detector to detector, particularly at the start of the record when the required correcti-on is large.

One of the approaches utilized in this continuous correction method is to mount the reproducing heads at spaced-apart points along a member which overlies the recording medium and which is pivoted at one end for programmed, nonlinear movement relative to the time axisof the recording medium. However, this method has the disadvantage that the different reproducing heads are spaced along the member at distances proportional to the'squares of the distances of the different detectors from the shot point, thus resulting in the reproducing heads corresponding to the detectors closest to the shot point being located at relatively closely spaced points near the xed end of the member, and the reproducing heads corresponding to the detectors farthest from the shot point being located at relatively widely spaced points recording in seismic prospecting, a number of methods near the movable end of the member. This is disadvantageous when it is considered that the reproducing heads nearest the xed end of the member are capable of relatively little movement unless the member is of almost prohibitively great length. This type of a system has the further disadvantage that, owing to the square law relationship existing between the detector spacing and reproducing head spacing, it is dilicult to accommodate changes in detector spacing. An additional disadvantage of this system is that it introduces the largest error at the early part of the seismic record when th'e required correction is also largest, thus resulting in a maximum error in the moveout correction operation.

Broadly, the present invention contemplates methods of and apparatus for correcting a plurality of seismic traces for the effects of normal moveout time variations in which the introduced correction varies nonlinearly along the traces with respect to time and nonlinearly from trace to trace.V More particularly, the invention contemplates introducing the desired correction by pro'- gramming the movementV of the reproducing, or recording. heads in accordance with the movement of a resilient member which has a deforming force of variable magnitude applied theretoduring; recording or reproduction; The resilient member is. suitablyv shaped so that its deformation in response tn the deforming force varies nonlinearly from head to head, corresponding` to thev required nonlinear correction fromY trace to trace, andy VariesY nonlinearly throughout the duration of the seismic traces, corresponding to the required. nonlinear variations inthe moveout correction along each trace. The heads are connected to different pointsv along the length of the member t-o varyv the positions of the different heads relative to each other and to the recording medium duringv the moveout correction operation. The reproducing or recording, heads are connected to the resilient member at points which are spaced in direct proportion to the distances of the different detectors from the shot point,-

so. that variations in the spacings of the detectors. may be readily accommodated by corresponding proportional variations. in the connections of the headsy to the resilient. member.

It; is, therefore, an object of this invention to provide improved methods of and apparatus for correcting a plurality of seismic traces for normal moveout time variations..

It is an additional object of the present invention to provide methods of and apparatus for correcting a plurality of seismic detector signals for normal moveout; time variations, in which the correction varies nonlinearly both throughout the duration of the traces and: from trace to trace.

It is a further object of this invention. to provide,` methods of and apparatus for correcting a plurality of reproducibly recorded seismic traces for normal moveout time variations in which the reproducing meansv associated with the reproducible recording medium .are disposed at spaced-apart points along a resilient member which is deformable relative to the recording medium.

It is an additional object of the present invention to provide methods of and apparatus for correcting a pluralityA of' reproducibly recorded seismic traces for nonlinear normalmoveout time variations in which the reproducing heads associated with the recording medium are disposed along the length of a resilient member which is nonlinearly deformed along its length during reproductionI of the traces to produce nonlinear variations inthe time sequences of reproduced traces.

It is an additional object of the present invention to provide methods of and apparatus for correcting a, pluralityY of reproducibly recorded seismic traces for nonlinear normal moveout time variations iny which the, reproducing heads associated with the recording medium are disposed along the length of a resilient, member which is nonlinearly deformed along its length during reproduction of the traces to produce nonlinear variar tions; in the time sequences 'of reproduced traces, the distances between the different heads along the resilient member being directly proportional to the distances between the different seismic wave detectors. which produce the different traces.l

It is an additional object of the present invention to provide methods of and apparatusy for correcting a plu- -rality of' reproducibly recorded seismic traces for nonlinea-r normal moveout time variationsin which thereproducing heads associated with the .recording medium are disposed along the length of a resilient member which has a deformation along its length during reproduction of thetraces which is nonlinear both along the traces and from trace to trace to produce nonlinear variations in thetime sequences of the reproduced traces.

Objects and advantages other than those sety forth above will be apparent from thel following description when read lin connection with the accompanying drawing; irtwhich.:v

Fig. 1 is a series of curves illustrating the effect of normal moveout time variations on a representative group of seismic detector signals;

Fig. 2 is a side View, partly in section, of one embodiment of a ,reproducible recording device for carrying out the present invention;v

Fig. 3 is a top view ofthe apparatusv illustrated in Fig. 2; and

Fig. 4 is a sectional View taken along plane IV--IV of F ig. 2.

To aid in understanding the considerations involved in normal moveout time variation corrections and the application of this. invention to this problem, Fig. l illustrates a series of curves representing a plurality of seismic traces obtained in a representative seismic prospecting operation. The different traces are identified as traces 16,r 17, 18, 19, 20', 21, 22 and. 23),v and each curve represents the output of a seismic detector plotted as afunc-V tion of time. the traces are spaced at equal distances along a line throughA the shot point, the detector corresponding to traceI 16 beingi located nearest. theV shot point. and the, detector corresponding to trace 23 being locatedV farthest froml the shot point.

In practice,l it Will be understood that in the split spread method' of seismic prospecting, another series of detectors similar to those producing traces 16 through, 23 will be, located in a line on the other side. of the shot point to produce a corresponding plurality of traces, which together with traces 16vv through 23,. give complete.

subterranean coverage as. the shot pointis moved, along( the line being surveyed. For the sake ofl simplicity, these other traces are not shown, but it. will be. understood that these traces. produced by these detectors would. be symmetrical withrcspect to traces 16 through 23 and. would be substantially the mirror image of these latter traces..

Traces 16' through4 23 each` have similar rst. peak, portions 16a, 17a, 18a, 19a, 20a, 21a, 22a and 23a representing reections of energy from a given shallow refleeting horizon. It will be noted that the positions of these first peaks arev shifted relative to each other along the time axis of the record, these time shifts corresponding to the normal moveout times for this reflection. It will be further noted that the time shifts between the peaks of the different traces follow a nonlinear pattern, aslahown by the dotted line 2,5 running through these pe s.

Each of the seismictraces. also has a. secondpeak 16b, 17b, 18h, 19h, 20h, 2lb, 22h and 23h at a. subsequent time in the record corresponding to receipt by the different detectors of energy from a second` reflecting horizon.

These peaks are shifted relative to each other along the time axis in a nonlinear fashion, although the total time shift between peaks 1Gb and 23h is not as great as it is for the first reflections represented by peaks 16a through 23a. Dotted line 26 running through peaks 16h through 23h illustrates that the normal, moveout correction for this particular reflection is also nonlinear and different from the moveout` correction represented by dotted line 25l for peaks 16a` through 23a. A third similar reflection 'is shown in each of the traces by peaks 16e through 23e, andi d'otted line 27 joining these peaks has less slope than either of' the preceding normal moveout functionlines 25 and 26, but nonnal moveout function 27 is also nonlinear. Additional subsequent common peaks in the records" and the corresponding dotted linetherethrough indicating the normal moveout function are illustrated in traces. 16. through 23 to,y show that the normal' moveout time variations decrease as the record progresses: until the normal moveout variations; are substantially' zero when lthe-differences in the distances of the travel paths` to the :the 'seismic record-l illustrated. in Fig.. 1 has. been idealized It is assumed that the detectors producing td 'show fairly pronounced peaks with a minimum of extraneous energy or noise, it will be understood that, in practice, considerable noise is usually present in seismic traces, and that visual alignment of the traces is usually quite difficult.

Assuming that a seismic record similarv to that illustrated in Fig. 1 has been produced by a plurality of seismic detectors arranged in the manner described, the methods and apparatus of this invention may be utilized to accurately remove the normal moveout time variations from such a record in the following manner. In Figs. 2, 3 and 4, reference numeral 31 designates generally a reproducible recording device on which the seismic traces may be recorded and then reproduced repeatedly and at will. Recording device 31 may comprise a rotor mounted inside a housing 35 and on which is disposed a recording medium in the form of a layer of magnetizable material 31a. The rotor on which recording medium 31a is disposed is driven by a motor 33 through a shaft 34.

Recorder 31 is further provided with a plurality of recording and/or reproducing heads 16', 17', 18', 19', 20', 21', 22, 23' having electrical connections to the different seismic detectors. These heads overlie recording medium 31a and are adapted to record and/or reproduce information on and from medium 31a. Each of heads 16 through 23 is movable in a slot in the recorder housing 35 a limited distance about the periphery of recording medium 31a to effect time shifts in the relative positions of these recording heads. Each of the recording heads is connected to a resilient or deformable bar member 41 whose movement is programmed in accordance with the normal moveout function obtaining in the area under survey. Member 41 may be disposed at a distance from the recording device and be connected to the different recording or reproducing heads through cables and pulleys, or member 41 may, as shown, have the recording or reproducing heads connected directly thereto and be disposed on recorder frame 35 to overlying medium 31a. Member 41 may be of any suitable material, such as stiff rubber or spring steel, capable of undergoing the required deformation with sufficient resiliency. One end of bar 41 is fixed to a mounting post 42 disposed in the center of recorder 31. The distance from the outer edge of post 42 to head 16' is preferably proportional to the distance between the shot point and the first detector, and the spacing between the heads 16 through 23' is similarly proportional to the spacings between the different detectors. The other encl of member 41 has applied thereto a deforming force which varies in magnitude during recording of the seismic traces to vary the conguration of member 41 during such recording. In the embodiment illustrated, the deforming force is applied by a camming finger or arm 43 secured to a disc 44 driven by motor 33 through a gearing 45 and a shaft 46. Different moveout -functions may be accommodated in resilient member 41 either through variations in the configuration of member 41, or, preferably, through variations in the point along the length of member 41 at which the deforming force is applied. It will be understood that for a resilient member of given length, the maximum curvature will occur when the deforming force is applied at the free end, and that correspondingly less curvature will occur as the point of application of the deforming force is moved toward the fixed end. In the connection, the reproducing or recording heads should be mounted between the fixed end and the point of application of the deforming force to obtain the desired programmed movement.

Heads 16 through `23 are suitably secured at spacedapart points along member 41 and are movable therewith so that the heads move about the periphery of recording medium 31a when deformable bar member 41 moves to effect time shifts in the positions of the different heads relative to the time axis of recording medium pla. The stiffness of member 41 and the force appliedl thereto by arm 43 are such as to overcome anytendency.j of the heads to bend or jam in the slots, thusinsuring that the heads move readily in accordance withv the de-I sired programming. The heads are secured to membery 41 in a manner which permits them to accurately track their associated channels on recording medium 31a without misalignment. Variations in the spacings of the detectors may be accommodated by utilizing members 41 of different lengths for the different detector spacings, so that the curvature of the selected member 41 corresponds to the moveout function for the particular detector spacing under consideration.

In operation, assume that seismic traces 16 through 23 have been recorded on medium 31a and that the moveout correction is to be accomplished during reproduction of the traces. As stated above, for the particular seismic surveying operation illustrated, another group of detectors would normally be utilized and their outputs would be similarly recorded on recorder 31 symmetrically about supporting member 42. .These latter heads are also connected at spaced-apart points along a deformable member similar to member 41 whose movement is programmed by a camming finger driven by motor 33. At the start of the reproducing operation, deformable members 41y would be deformed by cam fingers 43 to cause members 41 to assume the position shown in Fig. 2, with the curvature of members 41 at this point, as best shown in Fig. 3, corresponding to the curve of line 25 running through the first peaks 16a through 23a of seismic traces 16 through 23.

The operation of the invention may be more readily understood by considering that, in Fig. 1, the cylindrical surface of recording medium 31a is developed out into a plane surface with traces 16 through 23 recorded thereon and with curve 25, running through peaks 17a through 23a, corresponding to the curvature of resilient member 41 at the start of the reproducing operation. Each of heads 16 through 23 would thus overlie the associated peaks 16a through 23a of the respective seismic traces. Motor 33 may thereupon be started to start rotation of medium 31a under heads 16' through 23. As motor 33 drives recording medium 31a, camming finger 43 is also driven through gearing 45, shaft 46 and disc 44 to decrease the deforming force applied yto resilient member 41. Resilient member 41 thereupon starts to move to return to its neutral position, thus causing the heads 16 through 23 to move relative to each other and relative to the recording medium.

The movement of finger 43 and the configuration of resilient member 41 are so designed that when recording medium 31a has rotated to the corresponding peaks 16b through 23h, resilient member 41 has the configuration or shape shown by dotted curve 26 running through peaks, 16h through 23b. Heads 16 through 23 are thus accurately positioned relative to each other and to the recording medium 31a so that each of these heads overlies the portion of recording medium 31a containing the associated peaks 16h, 17h, lsb, 19h, 20b, 21b22b, and. 23h. As recording medium 31a continues to rotate, finger 43 continues to decrease the deforming force applied to resilient member 41, with consequent decrease in the curvature of this member. Thus, resilient member 41 assumes the shape shown by dotted curve 27 when peaks 16e through 23o appear on recording medium 31a, lso that heads 16 throuagh 23 exactly overlie the associated peaks 16e through 23c of the respective traces. The operation continues as the deforming force applied to resilient member 41 is decreased to reduce the curvature of resilient member 41 as the normal moveout function decreases during the duration of the seismic traces.

Although, for the purposes of illustration,.separate, definite movement functions 25, 26 and 27 have been illustrated, it will be understood that, in practice, the movement function will vary continuously throughout the duration of the seismic record and thatrthe configuration of resilient member 4I will correspondingly continuously vary during reproduction of the seismic traces so that the configuring of member- 41- at any instant has the value required; to position heads 16 through 23' at the correctl positions relative t'o each other and relative to7 recording' medium Zilla` to provide a continuous normal moveout correction.

Traces reproduced through heads 16 throughA 23', with:

normal moveout time variations removed, may be recorded on another recording medium or, alternatively, combined or mixed to produce a composite trace in which each of the reflections is emphasized relative tothe extraneous energy or noise.

As statedy above, the shape of resilient member 41' and the point atv which the deforming force is applied thereto maybe' varied'to accommodate different normal moveout functions, and the member shape and point of applicationof theI deforming force may be deterrnined from a consideration ofv the mechanics involved in the deflection of a cantilever beam in response toa deforming force; In general, it can be stated that, in the casey ofl a simplel beam which is xed atone end and which tapersto a point at its free end, if thel deforming force is applied at the freer end, the resulting curvature of thev member may closely' approximate the desired normal moveout' function, particularly if the curvature is not too pronounced. However, if additional curvature is required, the length ofthe taperedmember should be increased and the deforming force applied at a point' spaced from the free end, thus resulting in, elfectively, a resilient member whichapproximates a trapezoid along its length. It can be'shown that' thisla-tter member has the characteristic` that if its' maximum detlection is the same as that oa simple tapered beam of the same length', its deflection at any point along its length will always be greater than the deflection for the corresponding point along the length of the simple tapered beam. Thus, the trapezoidal shaped beam may be utilized to provide more curvature'for a givenA length than a simple tapered beam.

In this connection, if it is not possible to exactly match the beamV configuration to the normal moveout function throughout' the durationof the seismic record, the emphasis should be placed on obtaining maximum correlation at the start of the' seismicl record when the required normal moveout time correctionsV are substantially larger than-they are later in the record. Thus, on a percentage oferrorwbasis, an errorofSO percent in the introduced correction near the end ofl the seismic record, where the total required correction. may be only 2` milliseconds, will not be very4 serious', whereas, the same` percentage of.' error nearl the startk ofthe record, where the required correction may be, say, 50 milliseconds, will result in an appreciabley error in the corrected traces.

It will be seen that this invention provides novel methods andl apparatus forA removing normal moveout time variationsfrom a plurality of seismic traces'whichy permits the reproducingheads tol be spaced along the resilient member atdistances directly 'proportional to the distances between the different seismicwave detectors, so that variations in the detector spacings may be readily accommodated by proportional: variations in the positions of the heads along thek resilientmember. It" the detector spacings varyfrom record to' record and it is desired to' processa number of such records rapidly, itl may be preferable tov locate the resilientv deformable member at a distance from the recording medium, ratherv than have it as a partof the recorder housing itself; as inFigs. 2, 3 and 4,4 andi connect" theV heads to points along the member through a cable' and pulley arrangement; Changes in detector` spacing can then be easily accommodated by 'changing the points along the resilient member at which 'the differentcables are connected, thus facilitating changes inA theprogramming of the diferent'heads and permitting the use of a single resilientr member, for a plurality of different moveout functions;

AlthoughA but a` few: embodimentsy of. the present invention have been illustratedv and described, it'. will be apparent to those-skilled in: the art. that various'. changes-and Inodiic'ations may be made therein without departing: from the spirit of the invention or the scope ofi theap'- pended claims.

I; claim.:

l. Apparatus for" removing the eifectsof normal moveout on: the relative times: of' occurrence of corresponding. signal' portions of a plurality' of seismic detector traces comprising a reproducible recording medium for recording. sa-idtraces, aV plurality ofv reproducing heads movable relative to said mediumL for reproducing-v said traces, a detormable'4 member having a freey end to whichy a4 deforming force is applied and a fixed end, said deformable' member being nonlinearlydeformable along its' length in' response to said deforming4v force, means connecting' said reproducing heads at spaced-apartpointsl along the length of said member, and means for. varying the magnitude of said'. deforming forceJ applied to said free end of said member during reproduction ofl said. traces to nonlinearlfy vary'v the positions of saidreproducing heads relative to eachy other and tor said recording medium in accordance with. the normal moveout function to produce substantial time coincidence-of said corresponding signal portions of said reproduced traces.

2. Apparatus for' removing' theeifects of normal moveout on the relative tim-esi of occurrence of corresponding signal' portionsy of a plurality of seismic detector traces produced by a plurality of spaced seismic wave detectors comprising'a reproducible recording medium forl recordingsaid traces, a plurality of reproducing heads movable relative to said: medium for reproducing said traces, a deformable memberhaving al free endto which a deforming force is' applied and` a fixed end, said member being nonlinearlyV deformable along' its length in response to said deforming force, means connecting said; reproducing heads at spaced-apart points along the lengthl of said member, the spacingsy between said points being directly proportional to the spacings between said detectors, and means for varyingl the magnitude of said deforming force applied to said free end of said: member during reproduction of said traces to nonlinearly vary the positions of said reproducing heads relativer to each` other and to said recording: medium* in accordance with the normal moveout function toproduce substantial time coincidence of saidl corresponding signalr portions of said reproduced tracers.

3f. Apparatus for removing'the' effect of normal moveoutY on the relative times; of' occurrence of corresponding signal portions of a plurality' of seismic detector traces comprisinga rotor, a' reproducible recording. medium a'fisposedonl the periphery of said rotor for recordingsaid traces, al plurality of reproducing heads movable relative to said periphery [medium] for reproducing said' traces, a deformable member extending transversely of said periphery [recording medium]y and having a free end to which a deforming force isapplied and a ixed end, said member being nonlinearly deformable along itsV length in response to said deforming force, means connecting saidreproducing heads directly tol saidmember andv at `spaced-apart points along'thelength of said member, and

means for varying the magnitude of said deforming force applied to said free endv of said' member during reproduction of saidftraces to nonlinearly vary the positions' of saidV reproducing heads relative to each other and to said periphery [recording medium] in accordance with the normal moveout function toV produce substantial time coincidence of saidi corresponding signal portions of said reproduced traces.

4. Apparatus for removing the eliects ofy normal moveout onthe relativer times of occurrence of corresponding signal portions of' a plurality of' seismic detector traces produced by a` plurality' of'spaced seismic wavedetectors comprisingA a reproducible recording medium for recording said traces, a plurality of reproducing heads movable relative to said medium for reproducing said traces, a deformable member mounted to overlie directly said recording medium, said deformable member having a free end to which a deforming force is applied and a xed end, said mounting member being nonlinearly deformable along its length in response to said deforming force, [cable] means connecting said reproducing heads directly to spaced-apart points along the length of said member, the spacings of said points along said member being directly proportional to the spacings of said detectors, and means for varying the magnitude of said deforming force applied to said free end of said member during reproduction of said traces to nonlinearly vary the positions of said reproducing heads relative to each other and to said recording medium in accordance with the normal moveout function to'produce substantial time coincidence of said correspending signal portions of said reproduced traces.

5. Apparatus for removing the effects of normal moveout on the relative times of occurrence of corresponding signal portions of a plurality of seismic detector traces comprising a reproducible recording medium, a plurality of recording heads movable relative to said medium for recording said signals, a deformable member having a free end to which a deforming force is applied and a fixed end, said mounting member being nonlinearly deformable along its length in response to said deforming force, means connecting said recording heads at spaced-apart points along the length of said member, and means for varying the magnitude of said deforming force applied to said free end of said member during recording of said traces to nonlinearly vary the positions of said recording heads relative to each other and to said recording medium in accordance with the normal moveout function to produce substantial time coincidence of said corresponding signal portions of the recorded traces.

6. Apparatus for compensating for time-distance relationships between a plurality of seismic detector traces displaced relative to each other by the time difference for travel of seismic waves between a shot point, reflecting horizons and the laterally displaced locations of the plurality of seismic detectors generating said traces which comprises a rotary drum, a recording and reproducing surface disposed on the peripheral area of said drum, a plurality of reproducing heads mounted to overlie directly said recording surface on said drum, the mounting for said heads including a resilient cantilever beam having its fixed end mounted to permit a maior portion of said beam to overlie said drum and to permit the free end of said beam to be deected laterally relative to the axis of rotation of said drum, cam means engaging said free end of said cantilever beam and drive means operable in accordance with the rotation of said drum for varying the position of said beam relative to said recording and reproducing surface, said reproducing heads being mounted at spaced-apart points along the length of said beam so that during reproduction of said traces the positions of said heads relative to each other and said recording and reproducing surface are varied nonlinearly as the traces are positioned on said recording surface and thereby compensate for the time-distance relationships between said plurality of seismic detector traces.

References Cited in the iile of this patent or the original patent UNITED STATES PATENTS 2,243,730 Ellis May 27, 1941 2,427,421 Rieber Sept. 16, 1947 2,440,971 Palmer May 4, 1948 FOREIGN PATENTS 750,936 Germany Feb. 3, 1945 

